Download 120 Core Idea PS3 Energy PS3.A: DEFINITIONS OF

A Framework for K-12 Science Education: Practices, Crosscutting Concepts, and Core Ideas
By the end of grade 8. A stable system is one in which any small change results
in forces that return the system to its prior state (e.g., a weight hanging from
a string). A system can be static but unstable (e.g., a pencil standing on end).
A system can be changing but have a stable repeating cycle of changes; such
observed regular patterns allow predictions about the system’s future (e.g.,
Earth orbiting the sun). Many systems, both natural and engineered, rely on
feedback mechanisms to maintain stability, but they can function only within
a limited range of conditions. With no energy inputs, a system starting out in
an unstable state will continue to change until it reaches a stable configuration
(e.g., sand in an hourglass).
By the end of grade 12. Systems often change in predictable ways; understanding
the forces that drive the transformations and cycles within a system, as well as the
forces imposed on the system from the outside, helps predict its behavior under a
variety of conditions.
When a system has a great number of component pieces, one may not be
able to predict much about its precise future. For such systems (e.g., with very
many colliding molecules), one can often predict average but not detailed properties and behaviors (e.g., average temperature, motion, and rates of chemical
change but not the trajectories or other changes of particular molecules). Systems
may evolve in unpredictable ways when the outcome depends sensitively on the
starting condition and the starting condition cannot be specified precisely enough
to distinguish between different possible outcomes.
Core Idea PS3 Energy
How is energy transferred and conserved?
Interactions of objects can be explained and predicted using the concept of
transfer of energy from one object or system of objects to another. The total
energy within a defined system changes only by the transfer of energy into or
out of the system.
PS3.A: DEFINITIONS OF ENERGY
What is energy?
That there is a single quantity called energy is due to the remarkable fact that a
system’s total energy is conserved. Regardless of the quantities of energy transferred
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between subsystems and stored in various ways within the system, the total
energy of a system changes only by the amount of energy transferred into and
out of the system.
At the macroscopic scale, energy manifests itself in multiple phenomena,
such as motion, light, sound, electrical and magnetic fields, and thermal energy.
Historically, different units were introduced for the energy present in these different phenomena, and it took some time before the relationships among them were
recognized. Energy is best understood at the microscopic scale, at which it can be
modeled as either motions of particles or as stored in force fields (electric, magnetic, gravitational) that mediate interactions between particles. This last concept
includes electromagnetic radiation, a phenomenon in which energy stored in fields
moves across space (light, radio waves) with no supporting matter medium.
Motion energy is also called kinetic energy; defined in a given reference
frame, it is proportional to the mass of the moving object and grows with the
square of its speed. Matter at any temperature above absolute zero contains thermal energy. Thermal energy is the random motion of particles (whether vibrations
in solid matter or molecules or free motion in a gas), this energy is distributed
among all the particles in a system through collisions and interactions at a distance. In contrast, a sound wave is a moving pattern of particle vibrations that
transmits energy through a medium.
Electric and magnetic fields also contain energy; any change in the relative
positions of charged objects (or in the positions or orientations of magnets) changes the fields between them and thus the amount of energy stored in those fields.
When a particle in a molecule of solid matter vibrates, energy is continually being
transformed back and forth between the energy of motion and the energy stored
in the electric and magnetic fields within the matter. Matter in a stable form minimizes the stored energy in the electric and magnetic fields within it; this defines
the equilibrium positions and spacing of the atomic nuclei in a molecule or an
extended solid and the form of their combined electron charge distributions (e.g.,
chemical bonds, metals).
Energy stored in fields within a system can also be described as potential
energy. For any system where the stored energy depends only on the spatial configuration of the system and not on its history, potential energy is a useful concept
(e.g., a massive object above Earth’s surface, a compressed or stretched spring). It is
defined as a difference in energy compared to some arbitrary reference configuration of a system. For example, lifting an object increases the stored energy in the
gravitational field between that object and Earth (gravitational potential energy)
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compared to that for the object at Earth’s surface; when the object falls, the stored
energy decreases and the object’s kinetic energy increases. When a pendulum
swings, some stored energy is transformed into kinetic energy and back again into
stored energy during each swing. (In both examples energy is transferred out of the
system due to collisions with air and for the pendulum also by friction in its support.) Any change in potential energy is accompanied by changes in other forms of
energy within the system, or by energy
transfers into or out of the system.
Electromagnetic radiation (such
as light and X-rays) can be modeled as
a wave of changing electric and magnetic fields. At the subatomic scale (i.e.,
in quantum theory), many phenomena
involving electromagnetic radiation
(e.g., photoelectric effect) are best modeled as a stream of particles called photons. Electromagnetic radiation from
the sun is a major source of energy for
life on Earth.
The idea that there are different
forms of energy, such as thermal energy,
mechanical energy, and chemical energy,
is misleading, as it implies that the
nature of the energy in each of these manifestations is distinct when in fact they
all are ultimately, at the atomic scale, some mixture of kinetic energy, stored energy, and radiation. It is likewise misleading to call sound or light a form of energy;
they are phenomena that, among their other properties, transfer energy from place
to place and between objects.
Grade Band Endpoints for PS3.A
By the end of grade 2. [Intentionally left blank.]
By the end of grade 5. The faster a given object is moving, the more energy it possesses. Energy can be moved from place to place by moving objects or through
sound, light, or electric currents. (Boundary: At this grade level, no attempt is
made to give a precise or complete definition of energy.)
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❚ At the macroscopic scale, energy manifests itself in multiple
phenomena, such as motion, light, sound, electrical and magnetic fields,
and thermal energy.
❚
By the end of grade 8. Motion energy is properly called kinetic energy; it is proportional to the mass of the moving object and grows with the square of its speed.
A system of objects may also contain stored (potential) energy, depending on their
relative positions. For example, energy is stored—in gravitational interaction with
Earth—when an object is raised, and energy is released when the object falls or is
lowered. Energy is also stored in the electric fields between charged particles and
the magnetic fields between magnets, and it changes when these objects are moved
relative to one another. Stored energy is decreased in some chemical reactions and
increased in others.
The term “heat” as used in everyday language refers both to thermal energy
(the motion of atoms or molecules within a substance) and energy transfers by
convection, conduction, and radiation (particularly infrared and light). In science,
heat is used only for this second meaning; it refers to energy transferred when two
objects or systems are at different temperatures. Temperature is a measure of the
average kinetic energy of particles of matter. The relationship between the temperature and the total energy of a system depends on the types, states, and amounts
of matter present.
By the end of grade 12. Energy is a quantitative property of a system that depends
on the motion and interactions of matter and radiation within that system. That
there is a single quantity called energy is due to the fact that a system’s total energy is conserved, even as, within the system, energy is continually transferred from
one object to another and between its various possible forms. At the macroscopic
scale, energy manifests itself in multiple ways, such as in motion, sound, light,
and thermal energy. “Mechanical energy” generally refers to some combination of
motion and stored energy in an operating machine. “Chemical energy” generally is
used to mean the energy that can be released or stored in chemical processes, and
“electrical energy” may mean energy stored in a battery or energy transmitted by
electric currents. Historically, different units and names were used for the energy
present in these different phenomena, and it took some time before the relationships between them were recognized. These relationships are better understood at
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the microscopic scale, at which all of the different manifestations of energy can be
modeled as either motions of particles or energy stored in fields (which mediate
interactions between particles). This last concept includes radiation, a phenomenon in which energy stored in fields moves across space.
PS3.B: CONSERVATION OF ENERGY AND ENERGY TRANSFER
What is meant by conservation of energy?
How is energy transferred between objects or systems?
The total change of energy in any system is always equal to the total energy transferred into or out of the system. This is called conservation of energy. Energy cannot be created or destroyed, but it can be transported from one place to another
and transferred between systems. Many different types of phenomena can be
explained in terms of energy transfers. Mathematical expressions, which quantify
changes in the forms of energy within a system and transfers of energy into or out
of the system, allow the concept of conservation of energy to be used to predict
and describe the behavior of a system.
When objects collide or otherwise come in contact, the motion energy of one
object can be transferred to change the motion or stored energy (e.g., change in
shape or temperature) of the other objects. For macroscopic objects, any such process (e.g., collisions, sliding contact) also transfers some of the energy to the surrounding air by sound or heat. For molecules, collisions can also result in energy
transfers through chemical processes, which increase or decrease the total amount
of stored energy within a system of atoms; the change in stored energy is always
balanced by a change in total kinetic energy—that of the molecules present after
the process compared with the kinetic energy of the molecules present before it.
Energy can also be transferred from place to place by electric currents.
Heating is another process for transferring energy. Heat transfer occurs when two
objects or systems are at different temperatures. Energy moves out of higher temperature objects and into lower temperature ones, cooling the former and heating
the latter. This transfer happens in three different ways—by conduction within solids, by the flow of liquid or gas (convection), and by radiation, which can travel
across space. Even when a system is isolated (such as Earth in space), energy is
continually being transferred into and out of it by radiation. The processes underlying convection and conduction can be understood in terms of models of the possible motions of particles in matter.
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Radiation can be emitted or absorbed by matter. When matter absorbs
light or infrared radiation, the energy of that radiation is transformed to thermal
motion of particles in the matter, or, for shorter wavelengths (ultraviolet, X-ray),
the radiation’s energy is absorbed within the atoms or molecules and may possibly
ionize them by knocking out an electron.
Uncontrolled systems always evolve toward more stable states—that is,
toward more uniform energy distribution within the system or between the system and its environment (e.g., water flows downhill, objects that are hotter than
their surrounding environment cool down). Any object or system that can degrade
with no added energy is unstable. Eventually it will change or fall apart, although
in some cases it may remain in the unstable state for a long time before decaying
(e.g., long-lived radioactive isotopes).
Grade-Level Endpoints for PS3.B
By the end of grade 2. Sunlight warms Earth’s surface.
By the end of grade 5. Energy is present whenever there are moving objects,
sound, light, or heat. When objects collide, energy can be transferred from one
object to another, thereby changing their motion. In such collisions, some energy is
typically also transferred to the surrounding air; as a result, the air gets heated and
sound is produced.
Light also transfers energy from place to place. For example, energy radiated
from the sun is transferred to Earth by light. When this light is absorbed, it warms
Earth’s land, air, and water and facilitates plant growth.
Energy can also be transferred from place to place by electric currents,
which can then be used locally to produce motion, sound, heat, or light. The currents may have been produced to begin with by transforming the energy of motion
into electrical energy (e.g., moving water driving a spinning turbine which generates electric currents).
By the end of grade 8. When the motion energy of an object changes, there
is inevitably some other change in energy at the same time. For example, the
friction that causes a moving object to stop also results in an increase in the
thermal energy in both surfaces; eventually heat energy is transferred to the surrounding environment as the surfaces cool. Similarly, to make an object start
moving or to keep it moving when friction forces transfer energy away from it,
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energy must be provided from, say, chemical (e.g.,
burning fuel) or electrical (e.g., an electric motor
and a battery) processes.
The amount of energy transfer needed to
change the temperature of a matter sample by a
given amount depends on the nature of the matter,
the size of the sample, and the environment. Energy
is transferred out of hotter regions or objects and
into colder ones by the processes of conduction,
convection, and radiation.
By the end of grade 12. Conservation of energy means that the total change of
energy in any system is always equal to the total energy transferred into or out of
the system. Energy cannot be created or destroyed, but it can be transported from
one place to another and transferred between systems.
Mathematical expressions, which quantify how the stored energy in a system depends on its configuration (e.g., relative positions of charged particles, compression of a spring) and how kinetic energy depends on mass and speed, allow
the concept of conservation of energy to be used to predict and describe system
behavior. The availability of energy limits what can occur in any system.
Uncontrolled systems always evolve toward more stable states—that is,
toward more uniform energy distribution (e.g., water flows downhill, objects hotter than their surrounding environment cool down). Any object or system that
can degrade with no added energy is unstable. Eventually it will do so, but if the
energy releases throughout the transition are small, the process duration can be
very long (e.g., long-lived radioactive isotopes).
PS3.C RELATIONSHIP BETWEEN ENERGY AND FORCES
How are forces related to energy?
When two objects interact, each one exerts a force on the other. These forces can
transfer energy between the objects. Forces between two objects at a distance
are explained by force fields (gravitational, electric, or magnetic) between them.
Contact forces between colliding objects can be modeled at the microscopic level
as due to electromagnetic force fields between the surface particles. When two
objects interacting via a force field change their relative position, the energy in the
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force field between them changes. For any such pair of objects the force on each
object acts in the direction such that motion of that object in that direction would
reduce the energy in the force field between the two objects. However, prior
motion and other forces also affect the actual direction of motion.
Patterns of motion, such as a weight bobbing on a spring or a swinging
pendulum, can be understood in terms of forces at each instant or in terms of
transformation of energy between the motion and one or more forms of stored
energy. Elastic collisions between two objects can be modeled at the macroscopic scale using conservation of energy without having to examine the detailed
microscopic forces.
Grade Band Endpoints for PS3.C
By the end of grade 2. A bigger push or pull makes things go faster. Faster speeds
during a collision can cause a bigger change in shape of the colliding objects.
By the end of grade 5. When objects collide, the contact forces transfer energy so
as to change the objects’ motions. Magnets can exert forces on other magnets or
on magnetizable materials, causing energy transfer between them (e.g., leading to
changes in motion) even when the objects are not touching.
By the end of grade 8. When two objects interact, each one exerts a force on the
other that can cause energy to be transferred to or from the object. For example,
when energy is transferred to an Earth-object system as an object is raised, the
gravitational field energy of the system increases. This energy is released as the
object falls; the mechanism of this release is the gravitational force. Likewise, two
magnetic and electrically charged objects interacting at a distance exert forces on
each other that can transfer energy between the interacting objects.
By the end of grade 12. Force fields (gravitational, electric, and magnetic) contain
energy and can transmit energy across space from one object to another.
When two objects interacting through a force field change relative position,
the energy stored in the force field is changed. Each force between the two interacting objects acts in the direction such that motion in that direction would reduce
the energy in the force field between the objects. However, prior motion and other
forces also affect the actual direction of motion.
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PS3.D: ENERGY IN CHEMICAL PROCESSES AND EVERYDAY LIFE
How do food and fuel provide energy?
If energy is conserved, why do people say it is produced or used?
In ordinary language, people speak of “producing” or “using” energy. This refers
to the fact that energy in concentrated form is useful for generating electricity,
moving or heating objects, and producing light, whereas diffuse energy in the environment is not readily captured for practical use. Therefore, to produce energy
typically means to convert some stored energy into a desired form—for example,
the stored energy of water behind a dam is released as the water flows downhill
and drives a turbine generator to produce electricity, which is then delivered to
users through distribution systems. Food, fuel, and batteries are especially convenient energy resources because they can be moved from place to place to provide
processes that release energy where needed. A system does not destroy energy
when carrying out any process. However, the process cannot occur without energy
being available. The energy is also not destroyed by the end of the process. Most
often some or all of it has been transferred to heat the surrounding environment;
in the same sense that paper is not destroyed when it is written on, it still exists
but is not readily available for further use.
Naturally occurring food and fuel contain complex carbon-based molecules, chiefly derived from plant matter that has been formed by photosynthesis.
The chemical reaction of these molecules with oxygen releases energy; such reactions provide energy for most animal life and for residential, commercial, and
industrial activities.
Electric power generation is based on fossil fuels (i.e., coal, oil, and natural
gas), nuclear fission, or renewable resources (e.g., solar, wind, tidal, geothermal,
and hydro power). Transportation today chiefly depends on fossil fuels, but the
use of electric and alternative fuel (e.g., hydrogen, biofuel) vehicles is increasing.
All forms of electricity generation and transportation fuels have associated economic, social, and environmental costs and benefits, both short and long term.
Technological advances and regulatory decisions can change the balance of those
costs and benefits.
Although energy cannot be destroyed, it can be converted to less useful
forms. In designing a system for energy storage, for energy distribution, or to
perform some practical task (e.g., to power an airplane), it is important to design
for maximum efficiency—thereby ensuring that the largest possible fraction of
the energy is used for the desired purpose rather than being transferred out of the
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system in unwanted ways (e.g., through friction, which eventually results in heat
energy transfer to the surrounding environment). Improving efficiency reduces
costs, waste materials, and many unintended environmental impacts.
Grade Band Endpoints for PS3.D
By the end of grade 2. When two objects rub against each other, this interaction is
called friction. Friction between two surfaces can warm of both of them (e.g., rubbing hands together). There are ways to reduce the friction between two objects.
By the end of grade 5. The expression “produce energy” typically refers to the
conversion of stored energy into a desired form for practical use—for example,
the stored energy of water behind a dam is released so that it flows downhill
and drives a turbine generator to produce electricity. Food and fuel also
release energy when they are digested
or burned. When machines or animals
“use” energy (e.g., to move around),
most often the energy is transferred to
heat the surrounding environment.
The energy released by burning fuel
or digesting food was once energy from
the sun that was captured by plants in the
chemical process that forms plant matter
(from air and water). (Boundary: The fact
that plants capture energy from sunlight is
introduced at this grade level, but details
of photosynthesis are not.)
It is important to be able to concentrate energy so that it is available for use
where and when it is needed. For example, batteries are physically transportable
energy storage devices, whereas electricity generated by power plants is transferred
from place to place through distribution systems.
By the end of grade 8. The chemical reaction by which plants produce complex
food molecules (sugars) requires an energy input (i.e., from sunlight) to occur. In
this reaction, carbon dioxide and water combine to form carbon-based organic
molecules and release oxygen. (Boundary: Further details of the photosynthesis
process are not taught at this grade level.)
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Both the burning of fuel and cellular digestion in plants and animals involve
chemical reactions with oxygen that release stored energy. In these processes, complex molecules containing carbon react with oxygen to produce carbon dioxide
and other materials.
Machines can be made more efficient, that is, require less fuel input to perform a given task, by reducing friction between their moving parts and through
aerodynamic design. Friction increases energy transfer to the surrounding environment by heating the affected materials.
By the end of grade 12. Nuclear fusion processes in the center of the sun release
the energy that ultimately reaches Earth as radiation. The main way in which that
solar energy is captured and stored on Earth is through the complex chemical process known as photosynthesis. Solar cells are human-made devices that likewise
capture the sun’s energy and produce electrical energy.
A variety of multistage physical and chemical processes in living organisms,
particularly within their cells, account for the transport and transfer (release or
uptake) of energy needed for life functions.
All forms of electricity generation and transportation fuels have associated
economic, social, and environmental costs and benefits, both short and long term.
Although energy cannot be destroyed, it can be converted to less useful
forms—for example, to thermal energy in the surrounding environment. Machines
are judged as efficient or inefficient based on the amount of energy input needed
to perform a particular useful task. Inefficient machines are those that produce
more waste heat while performing a task and thus require more energy input. It is
therefore important to design for high efficiency so as to reduce costs, waste materials, and many environmental impacts.
Core Idea PS4Waves and Their Applications in Technologies for Information
Transfer
How are waves used to transfer energy and information?
Waves are a repeating pattern of motion that transfers energy from place to place
without overall displacement of matter. Light and sound are wavelike phenomena.
By understanding wave properties and the interactions of electromagnetic radiation with matter, scientists and engineers can design systems for transferring information across long distances, storing information, and investigating nature on
many scales—some of them far beyond direct human perception.
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